1. Field of the Invention
This invention relates to chemical compositions for generating large volumes of gas. More particularly, a mixture of nitroguanidine, ammonium nitrate, potassium nitrate and an elastomeric binder is ignited and the gaseous combustion products used to inflate an automotive airbag.
2. Description of the Prior Art
Airbags, as a component of a passive automobile restraint system, are installed in the steering column and passenger side dashboard of passenger automobiles. The airbags inflate in a collision and, by restraining the passengers, minimize injury.
Typically, sensors mounted in the automobile detect a collision and send an electric signal igniting a chemical mixture that generates a large quantity of gas during deflagration. This gas is used to deploy the airbag.
As disclosed in U.S. Pat. No. 3,797,854 to Poole et al., which is incorporated by reference in its entirety herein, one common chemical mixture contains an azide, such as sodium azide, and an inorganic oxidizer, such as potassium perchlorate.
Sodium azide is difficult to handle safely and it is toxic. Assembly of the airbags must be done in a controlled environment and disposal of undeployed airbag cylinders is difficult.
The search for a replacement for an azide/inorganic oxidizer composition to inflate airbags has lead to the identification of five targets for the ideal chemical mixture.
(1) The chemical mixture should generate a large volume of benign gases with minimal generation of noxious gases such as carbon monoxide (CO) and nitrogen oxides (NO.sub.x). One problem with azide based compositions is a low gas output, typically less than 1.5 moles of gas per 100 grams of the mixture. Azide alternatives can provide a significant increase in gas output, typically through the addition of CO.sub.2 and H.sub.2 O to the exhaust. The co-generation of CO and NO.sub.x is limited by proper selection of propellant composition and proper combustion.
(2) The chemical mixture must be thermally stable at temperatures in excess of 100.degree. C. Automobiles may remain in service for many years and are subject to temperature extremes. The gas generating composition must have a working temperature in the range of from about -40.degree. C. to about 100.degree. C. The chemical compounds when heated to a temperature of 100.degree. C. should not exhibit a significant net weight loss nor any evidence of physical change.
(3) The generation of solids is detrimental. The solids do not assist in the inflation of the airbag and must be filtered from the gas stream.
(4) The flame temperature or the combustion temperature of the chemical mixture should be as low as possible. At lower temperatures, decreased levels of CO are generated due to formation of more carbon dioxide. Lower levels of NO.sub.x are generated because of more favorable equilibrium and kinetic considerations.
(5) The chemical mixture should be deflagrating as opposed to detonating. On ignition, the mixture should burn rapidly rather than explode.
One substitute for azide/inorganic oxidizer gas generating mixtures is a mixture of 5-aminotetrazole and strontium nitrate plus other additives as disclosed in U.S. Pat. No. 5,035,757 to Poole. These compositions typically have greater gas outputs than azide generating gas compositions and exhibit good thermal stability. However, the flame temperature exceeds 2500K resulting in excessively high levels of CO and NO.sub.x. Furthermore, although toxicity concerns are considerably reduced, as compared to azide propellants, gas output levels are limited by the high levels of solids in the exhaust composition.
As disclosed in copending and commonly assigned U.S. Pat. No. 5,538,567 entitled "Gas Generating Propellant" by Henry, III et al. and is incorporated by reference in its entirety, one category of gas evolving compounds includes a guanidine salt. Gas is generated by igniting a mixture consisting essentially of (by weight) 55%-75% guanidine nitrate, 25%-45% of an oxidizer selected from the group consisting of potassium perchlorate and ammonium perchlorate, 0.5%-5% of a flow enhancer and up to 5% of a binder.
The mixture disclosed in Henry et al. is for use an augmented airbag system. In augmented systems, the main use of the propellant is to heat a pressurized gas which is the primary gas source for inflation of the bag. The amount of gas produced by the propellant is a small fraction of the total gas required to inflate the airbag.
An extrudable, non-azide based, propellant is disclosed in U.S. Pat. No. 5,125,684 to Cartwright. This propellant contains from about 45-80 wt. % of an oxidizer salt; an effective amount of a cellulose based binder; and from about 10-35 wt. % of at least one energetic component.
A nitrocellulose binder is not particularly favored for propellants intended for automobile airbag applications because of its poor chemical stability at the high temperatures experienced in the automobile environment. Additionally, the nitro (NO.sub.2 .cndot.) groups of the nitrocellulose contribute to the formation of higher levels of NO.sub.x during combustion.
Ammonium nitrate (AN) based propellants offer the capability of meeting many of the targets for airbag inflation. Many AN-based propellants and explosives are known.
German Patentschrift 851,919, published October 1952 by Imperial Chemicals Industries Limited, discloses a gas generating compound containing ammonium nitrate, sodium nitrate, guanidine nitrate and nitroguanidine.
U.S. Pat. No. 4,421,578 by Voreck, Jr., discloses an explosive mixture containing ammonium nitrate, potassium nitrate, nitroguanidine and ethylenediamine dinitrate. This composition was developed for explosive applications with an intent to replace TNT (2,4,6-trinitrotoluene). The eutectic formed when ammonium nitrate, ethylene diamine dinitrate and guanidine nitrate are mixed in the disclosed proportion has a melting temperature below 100.degree. C. Propellant mixtures with such a low melting point are not suitable for applications such as automobile airbag inflators where temperature stability in excess of 107.degree. C. is frequently required.
Both ammonium nitrate and phase stabilized ammonium nitrate (PSAN) are thermally stable for extended periods of time at a temperature of 107.degree. C. However, mixtures of AN and PSAN with a wide variety of materials ranging from polymeric binders to high energy fuels to common burn rate catalysts do not exhibit acceptable thermal stability as measured by weight loss and/or melting. Table 1 illustrates this phenomenon.
TABLE 1 ______________________________________ Oxidizer Fuel Additive Weight Loss* ______________________________________ PSAN None None .ltoreq.0.1% PSAN Hydroxy-terminated Milori blue, 0.5% polybutadiene (HTPB)/ IPDI PSAN None Milori blue 4% PSAN None Carbon black 35% PSAN Hydroxy-terminated Milori blue 6% polycarbonate-IPDI PSAN 5-amino-tetrazole None Melts with loss of NH.sub.3 PSAN ethylene diamine potassium Melts dinitrate, nitroguanidine nitrate &lt;100.degree. C.** ______________________________________ Table 1 notes: *= After thermal aging 400 hours at 107.degree. C. IPDI = isophorone diisocyanate Milori blue = an iron blue pigment. **= coposition of U.S. Pat. No. 4,421,578.
A problem with the use of pure ammonium nitrate is that the compound undergoes a series of structural phase transformations over the typical operating range of automobile airbag inflators. In pure AN, structural phase transitions are observed at -18.degree. C., 32.3.degree. C., 84.2.degree. C. and 125.2.degree. C. The phase transition at 32.3.degree. C. is particularly problematic during temperature cycling because of a large change in the associated volume, on the order of 3.7%, by volume. Generally, any volumetric change is detrimental and it is desired to limit any volumetric change as much as possible.
Phase stabilization of ammonium nitrate by the inclusion of potassium salts, such as potassium nitrate and potassium perchlorate is known. PSAN containing 15% by weight potassium nitrate will successfully avoid the problematic phase changes and volume changes associated with pure AN.
There remains therefore a need for an azide-free chemical composition useful to inflate automotive airbags that generates large volumes of benign gases, has thermal stability at temperatures in excess of 100.degree. C., generates a low volume of solids, has a low flame temperature and is not explosive.